Traditionally, solar cells have had difficulty in enduring extended exposure to weather elements, such as rain, snow, sleet, hail, and heat. Many attempts have been made to extend the life of solar cells by covering a solar cell or string of solar cells in anti-reflection or index-matching layers and by encasing a solar cell or string of solar cells in water resistant and weather impervious encapsulants.
U.S. Pat. No. 4,262,161 discloses a solar cell assemblage prepared using a transparent film comprised of a block copolymer having at least two monoalkenyl arene polymer end blocks A and at least one polymer mid block B selected from the group consisting of substantially completely hydrogenated conjugated diene polymer blocks, ethylenepropylene polymer blocks, and ethylene-butene polymer blocks between the cover plate and the electrical contact of the solar cell.
Even though this film provides good weather resistance for the resultant solar cell assemblage, it has been long desired to develop a novel film for use in solar cell assemblages which (1) is transparent to sunlight; (2) resistant to ultraviolet degradation; (3) has a refractive index intermediate that of glass and an anti-reflection coating on the solar cell; (4) has low water absorption/permeation properties; (5) acts as an electrical insulator; (6) is heat sealable; (7) has a wide service temperature range, preferably from about -40.degree. C. to about 90.degree. C.; (8) has good adhesion, giving a peel strength of up to 40 pounds per inch with cohesive failure, and (9) can be extruded or curtain coated onto suitably prepared glass or silicone-based substrates.
Known films used in solar cell assemblages have had a specific disadvantage; they have been highly susceptible to delamination because they have had low peel strength, in the range of less than 10 pounds per inch, with adhesive failure. A need has long existed to prepare a solar cell with a film capable of achieving a peel strength of over 10 pounds per inch with cohesive, not adhesive, failure.
While polyvinylbutyral (PVB) and polyethylene/vinylacetate (EVA) are the present encapsulants which are standard in the photovoltaic industry, they are not ideal candidates. PVB is hygroscopic and thus requires careful control of temperature and humidity during storage and conditioning and during sheeting operations in a manufacturing environment. Failure to control the atmosphere results in poor laminations due to voids formed from outgassing of absorbed water. The moisture sensitivity of PVB also dictates the use of a metal foil backskin which is unacceptable in several product specifications. EVA, on the other hand, has a requirement for peroxide catalyzed crosslinking to achieve creep resistance, a fact that presents several concerns. Since the peroxide in the film is relatively volatile, its concentration in the film in free sheet form, which determines the ultimate level of crosslinking, can vary as described in I. E. du Pont de Nemours & Co., Technical Guide, Elvax 150 EVA-Solar Photovoltaic Module Pottant, J. D. Pomije. Thus, the degree of crosslinking can vary and there is no good test of this parameter in an in-line production process. A second concern is that the time to achieve sufficient crosslinking is not consistent with a high throughput production lamination cycle time. Thirdly, while peroxide is volatile in free-standing film, it is not so volatile that all unused peroxide is removed during lamination, which could compromise the longevity of some module components.
The problems alluded to have led to the search for an alternative pottant that is not only easily controlled in a high throughput manufacturing setting, but is at least as reliable as PVB and EVA.
U.S. Pat. No. 3,984,369 discloses a composition which could be used as a sealant and which has good substrate/sealant adhesion, good UV stability, good peel strength in tests against glass, and useful hardness, tensile, and flexibility properties. This novel sealant comprises:
(a) about 3 to 30 percent by weight of an elastomeric ABA type poly(.alpha.-monoalkenyl arene)/hydrogenated poly(conjugated diene) block copolymer having at least two poly(.alpha.-monoalkenyl arene) A blocks wherein the average molecular weight of the arene blocks is 4,000 to 50,000; the average molecular weight of the poly(conjugated diene) B block is 18,000 to 250,000; the conjugated diene block contains at least 20% 1,2 bonds prior to hydrogenation; and at least 98% of the double bonds present in the conjugated diene blocks are saturated during hydrogenation;
(b) about 2 to 40 percent by weight of butyl rubber;
(c) about 1 to about 70 percent by weight of an oil having a solubility parameter of from about 6 to about 8;
(d) about 1 to about 50 percent by weight of an adhesion promoting resin having a solubility parameter of from about 8 to about 12;
(e) about 0 to about 70 percent by weight of an inorganic filler; and
(f) about 0.01 to about 2.0 percent by weight of an oxidation stabilizer.
Even though this composition has good UV stability and does not undergo covalent crosslinking, the adhesion of the compositions containing little or no butyl rubber was poor, giving only low peel strength and failing by the undesirable adhesive failure mechanism. The reason for this is that the ABA type polymers are very strong, making it impossible to achieve a composition which will fail by the desired cohesive failure mechanism. Compositions based on an ABA type polymer can be quite high in viscosity, making them difficult to extrusion coat onto a substrate. An alternative method of applying these high viscosity compositions is to mix the composition in an extruder, grind it to form a powder, apply it to the desired object, and then heat fuse the powder onto the object forming an encapsulant coating. One such process is described in U.S. Pat. No. 4,207,369.
Another novel composition, taught in U.S. Pat. No. 4,296,008, involved a sealant composition with improved adhesion and melt flow properties based on the lower strength and lower viscosity A'B'/ABA type block polymer. This second sealant composition was taught which comprises:
(a) 100 parts by weight of a selectively hydrogenated block copolymer component comprising an A'B' block copolymer and an ABA type multiblock copolymer having at least two end blocks A and at least one mid block B wherein the A' and A blocks are monoalkenyl arene polymer blocks and the B' and B blocks are substantially completely hydrogenated conjugated diene polymer blocks, the number average molecular weight of the A' and A blocks are between about 3,000 and about 7,000 and the monoalkenyl arene content of the multiblock copolymer is between about 7% and about 22% by weight, wherein the weight ratio between the A'B' block copolymer and the ABA type multiblock copolymer is from about 20:80 to about 60:40;
(b) about 50 to about 350 parts by weight of a tackifying resin compatible with block B;
(c) about 0 to about 100 parts by weight of a plasticizer; and
(d) about 0.1 to about 10 parts by weight of a silane coupling agent.
To insure good adhesion between a pottant based on an ABA type block copolymer and a substrate, the pottant must contain an adhesion promoting tackifying resin. To maintain good adhesion and impact resistance in the solar cell at low temperatures, the pottant must have a low glass transition temperature (Tg). The Tg of the hydrogenated polybutadiene B block of the block copolymer is -58.degree. C. by differential scanning calorimetry (DSC). The Tg of a typical 95.degree. C. softening point tackifying resin is about 45.degree. C. The Tg of the pottant comprised of a blend of ABA type block copolymer and tackifying resin will be intermediate between the Tg's of the polymer and the resin and will depend on the relative proportions of polymer and resin in the blend. If it is necessary to maintain low Tg in the pottant (e.g. -30.degree. C.), only a limited amount of the high Tg resin can be included in the composition. The relatively high percentage of ABA type block copolymer required in the composition to insure low Tg causes the melt viscosity of the pottant to be very high, making it difficult to extrusion coat the pottant onto a substrate.
To overcome this problem of high viscosity, it was discovered that the block copolymer used in the pottant must be of the ABA/A'B' type. The A'B' type diblock copolymers have much lower melt viscosity than do the ABA type multiblock copolymers. It was found that by using a block copolymer containing about a 30/70 ratio of ABA/A'B' block copolymers, a pottant could be made containing a limited amount of tackifying resin which had good adhesion and low Tg and which could be readily extrusion coated onto a substrate. Even though the novel composition could be extruded and applied to a variety of substrates such as glass or silicone release coated paper, the discovery of this novel composition was not enough to enhance the development of solar cells in general.
A silane treatment was developed to be used in combination with this novel extrudable pottant composition. The development of a silanization procedure that adequately promoted the adhesion of this novel composition to substrates in solar cells through prolonged humidity and freezing cycles was a critical discovery in the solar cell art. This discovery resulted in a method for preparing solar cells with improved durability.
A durable, mass-producible and cost-effective solar cell assemblage has long been sought. To develop such an array, a durable, mass-producible and cost-effective polymer frame has been developed. It is desirable to have as this frame, a thin, elastomeric, cohesive film that is highly flexible at low and high temperatures as well as being heat sealable.
More specifically, a polymeric encapsulant has long been needed for solar cells which is capable of the following traits:
(a) reducing manufacturing costs as compared to current polymer frames; PA0 (b) providing support structure for corner mounting of arrays with estimated minimum material properties of a 60.degree. C. modulus of .about.500,000 psi; PA0 (c) passing all UL and JPL test specifications including: PA0 (d) having side rail bow of no more than 1/8" in any direction; PA0 (e) sustaining adhesion to a substrate when subjected to pull of twice the module weight; PA0 (f) enduring up to 30 years of North American weather with less than 5% loss in mechanical properties and maintaining UV stability for an "acceptable" appearance; PA0 (g) maintaining a moisture barrier of edge seal at the substrate surface of less than 0.003 metric per cm; PA0 (h) providing a weight comparable to an extant aluminum frame; PA0 (i) having utility in a production environment; PA0 (j) having intra-array wiring molded into the walls of the frame; PA0 (k) having Class A residential housing flammability resistance; PA0 (l) capable of sustaining a single mounting position at the center of a solar array; PA0 (m) having a lighter weight than current frame systems; and PA0 (n) providing electrical insulation. PA0 (I) a solar cell assemblage comprising: PA0 (II) a method for making a solar cell assemblage comprising:
1. thermal cycle PA1 2. humidity freeze cycle PA1 3. mechanical loading PA1 4. twisted-mounting surface PA1 5. hail-impact test procedures PA1 (a) a pre-treated silanized transparent substrate; PA1 (b) a layer of semiconductor material; PA1 (c) an electrical contact on the surface of said layer of semiconductor material; PA1 (d) an extruded transparent film encapsulating the assemblage comprising (a), (b) and (c), said film comprising about 75 to 91 parts by weight of a blend of: PA1 about 65 to about 75 parts by weight of a selectively hydrogenated two-block polymer wherein one polymer block is designated by A and a second polymer block is designated by B such that prior to hydrogenation, PA1 about 25 to about 35 parts by weight of a selectively hydrogenated multiblock copolymer which contains at least two kinds of polymer blocks wherein one polymer block is designated by A and a second polymer block is designated by B such that: PA1 about 8.5 to about 23.5 parts by weight of an (alpha) methyl styrene polymer tackifying resin; and PA1 about 0.5 to about 1.5 parts by weight of a mixture of a phenolic antioxidant, a UV absorber with benzotriazole functionality, and a UV absorber with hindered amine functionality; PA1 (e) a carbon filled polymer frame to contain said encapsulated solar cell laminate; and PA1 (f) sealant to seal the frame to the solar cell laminate; and PA1 (a) silanizing a substrate thereby forming a silanized substrate; PA1 (b) extrusion coating one side of said silanized substrate with a first thin extruded coherent film, thereby forming a coated substrate, said first film consisting essentially of: PA1 (c) extrusion coating a backskin on the other side of said silianized substrate; PA1 (d) disposing on said coated substrate a layer of semiconductor material; PA1 (e) disposing an electrical contact on the surface of said semiconductor material; PA1 (f) coating said electrical contact with a second thin extruded coherent film consisting essentially of the same blend composition as the first film and forming an assemblage; PA1 (g) disposing a cover on said assemblage; PA1 (h) applying a vacuum to the assemblage thereby forming a pressure gradient between said substrate and said films, and thereby vacuum laminating the assemblage; PA1 (j) disposing said heat sealed assemblage in a carbon-filled frame; and PA1 (k) sealing said frame with an essentially rubber-type sealant and thereby forming a durable solar cell array.
The present inventive solar cell assemblage was intended to provide a solar cell assemblage with a polymeric encapsulant which can be thermoset and injection molded. An object of the instant invention was to support a commercial 40 watt solar cell module supported at 4 mounting points molded into the polymeric encapsulant without any center mounting. Another object of the instant invention was to provide a polymeric encapsulant with panels of a thickness between about 0.210".+-.0.020". Yet another object of the present invention was to provide a polymeric encapsulant capable of containing intra-panel wiring harnesses. It was also an object of the invention to provide a solar cell assemblage capable of supporting a protective outer coating, such as a gel coat which improves a polymeric encapsulant's weatherability. It was an object of the present invention to provide polymeric encapsulants having a maximum mold temperature of about 90.degree. C. and a button-to-button cycle time of less than 10 minutes.